Nullable Non Nullable Types

Kotlin null safety is a procedure to eliminate the risk of null reference from the code. Kotlin compiler throws NullPointerException immediately if it found any null argument is passed without executing any other statements.
Kotlin's type system is aimed to eliminate NullPointerException form the code. NullPointerException can only possible on following causes:

• An forcefully call to throw NullPointerException();
• An uninitialized of this operator which is available in a constructor passed and used somewhere.
• Use of external java code as Kotlin is Java interoperability.

Kotlin Nullable Types and Non-Nullable Types

Kotlin types system differentiates between references which can hold null (nullable reference) and which cannot hold null (non null reference). Normally,types of String are not nullable. To make string which holds null value, we have to explicitly define them by putting a ? behind the String as: String?

Nullable Types

Nullable types are declared by putting a ? behind the String as:

var str1: String? = "hello"  
str1 = null // ok  
var str1: String? = "hello"  
str1 = null // ok 

fun main(args: Array< String>){  
var str: String? = "Hello" // variable is declared as nullable  
str = null  
    print(str)  
}  
fun main(args: Array< String>){  
var str: String? = "Hello" // variable is declared as nullable  
str = null  
    print(str)  
}  			  

null

Non Nullable Types

Non nullable types are normal strings which are declared as String types as:

val str: String = null // compile error  
str = "hello" // compile error Val cannot be reassign  
var str2: String = "hello"  
str2 = null // compile error  
val str: String = null // compile error  
str = "hello" // compile error Val cannot be reassign  
var str2: String = "hello"  
str2 = null // compile error  

What happens when we assign null value to non nullable string?

fun main(args: Array< String>){  
var str: String = "Hello"  
str = null // compile error  
    print(str)  
}  
fun main(args: Array< String>){  
var str: String = "Hello"  
str = null // compile error  
    print(str)  
}  

It will generate a compile time error.

Error:(3, 11) Kotlin: Null can not be a value of a non-null type String

Checking for null in conditions

Kotlin's If expression is used for checking condition and returns value.

fun main(args: Array< String>){  
var str: String? = "Hello"     // variable is declared as nullable  
var len = if(str!=null) str.length else -1  
println("str is : $str")  
println("str length is : $len")  
  
str = null  
println("str is : $str")  
len = if(str!=null) str.length else -1  
println("b length is : $len")  
}  
fun main(args: Array< String>){  
var str: String? = "Hello"     // variable is declared as nullable  
var len = if(str!=null) str.length else -1  
println("str is : $str")  
println("str length is : $len")  
  
str = null  
println("str is : $str")  
len = if(str!=null) str.length else -1  
println("b length is : $len")  
}  			  

str is : Hello
str length is : 5
str is : null
b length is : -1